Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the fuel vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the fuel vapors to be purged into the engine intake manifold from the fuel vapor canister. The fuel vapors are then consumed during combustion. In addition to canister fuel vapors, positive crankcase ventilation (PCV) fuel vapors may also be ingested and combusted in the engine during engine operation. By recycling the fuel vapors, engine fuel economy is improved.
Engine systems may also utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. EGR effectively cools combustion chamber temperatures thereby reducing NOx formation. Also, EGR reduces pumping work of an engine resulting in increased fuel economy.
One common issue with the purging of crankcase and canister hydrocarbons to an engine intake is the control of a combustion air-fuel ratio. In particular, due to misdistributions introduced by the ingested vapors, as well as large discrepancies in the estimation of fuel vapor concentrations from the canister and the crankcase, it may be difficult to control the air-fuel ratio of the cylinders where the vapors are introduced for combustion. As such, the air fuel imbalance can lead to degraded engine performance and elevated exhaust emissions. Further, the ingestion and combustion of the fuel vapor hydrocarbons can result in poor engine NVH characteristics. As a result, purge flow and crankcase ventilation flow levels may be limited to improve drivability and passenger comfort. Purge and crankcase ventilation efficiency may also be limited due to the presence of EGR flow. For example, when the engine is operating with EGR at higher levels, there may be insufficient manifold vacuum to draw in the canister and crankcase fuel vapors. Consequently, the engine's ability to leverage purge flow and PCV for attaining higher fuel economy is compromised.
The inventors herein have recognized that there may be conditions where drivability is reduced due to causes external to the vehicle, such as due to rough road conditions. During such conditions, the flow of purge and PCV vapors may be opportunistically raised since the operator may be unable to distinguish any NVH issues associated with the increased hydrocarbon ingestion (and combustion) levels from those associated with the rough road condition. In other words, any combustion instability generated by the increased purge flow can be better masked by the increased NVH from the rough road. EGR levels may also be opportunistically increased during the rough road conditions and coordinated with purge/PCV flow so as to provide sufficient manifold vacuum for drawing in the fuel vapors. The fuel economy potential of purge and/or PCV usage can be improved using an example method for an engine comprising: in response to an indication of road roughness, selectively adjusting one or more engine operating parameters to increase fuel economy, the selectively adjusting including transitioning from a first level associated with lower NVH and combustion instability to a second level associated with higher NVH and combustion instability. For example, the method may include selectively increasing a flow rate of fuel canister hydrocarbons and/or crankcase ventilation hydrocarbons being purged to the engine.
As an example, road roughness conditions may be monitored using input from a plurality of sensors. These may include, as non-limiting examples, crankshaft acceleration sensors, wheel speed sensors, dynamic suspension system sensors including yaw-rate sensors, steering wheel sensors, etc. During smoother road conditions, purge and/or PCV flow may be lowered from a target level in order to reduce potential NVH issues associated with higher hydrocarbon ingestion levels. In comparison, during rougher road conditions, purge and/or PCV flow may be opportunistically raised to or above the target level. Likewise, during rough road conditions, EGR flow may also be opportunistically raised. However, a degree of increasing the EGR level may be limited based on the intake manifold vacuum level. Thus, when the available intake manifold vacuum level is higher, while purge and/or PCV flow is increased, EGR flow may be increased to a higher degree. In comparison, when the available intake manifold vacuum level is lower, while purge and/or PCV flow is increased, EGR flow may be increased to a lower degree (or not increased) so as to provide sufficient manifold vacuum for the purge and/or PCV flow.
It will be appreciated that one or more additional engine operating parameters that improve fuel economy but cause increased NHV or combustion instability at higher levels may also be concurrently adjusted during the rough road conditions. For example, transmission gear shift schedule may be advanced so that the gear shift can be completed during the rough road conditions. As another example, less spark retard may be used during the gear shift schedule. As yet another example, a torque convertor lock-p clutch may be slipped less during the gear shift schedule. Further still, during the rough road condition, while purge flow and EGR flow is elevated, knock and pre-ignition thresholds may be adjusted to correspond to thresholds that have more spark advance. Likewise, exhaust cam phasing in a variable cam timing (VCT) engine may be adjusted.
In this way, by increasing one or more parameters such as fuel vapor canister purge frequency, positive crankcase ventilation flow, EGR delivery, use of spark retard during gear shift schedule, torque convertor slip schedule and exhaust cam phasing, etc., during conditions of elevated road roughness, higher engine fuel economy may be achieved without an additional increase in NVH for the vehicle occupants. By enabling NVH associated with elevated ingestion and combustion of fuel vapor hydrocarbons (such as from elevated purge levels, for example) to be better masked by NVH associated with rough road conditions, a higher fuel vapor usage may be enabled, improving engine performance. By increasing purging frequency, fuel vapor canister cleaning efficiency over a vehicle drive cycle is improved. As such, the increased fuel vapor usage during rough road conditions may be particularly advantageous in global markets where road conditions are generally poor. The technical effect of adapting engine operating parameters responsive to rough road conditions to improve fuel economy is that higher fuel economy and improved emissions benefits may be achieved without any noticeable increase in NVH.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.